Valide Invite to Vote

Some people can't access the poll or vote multiple times and some can. The people who can't vote twice have the following message : "pour voter vous avez besoin d'une invitation valide et non-utilisée" which translate to "To vote you need a valid invite or an unused invite". How can they access the survey and vote more than once ?

Hi,
Can you send the link to your contest to support@pollunit.com?
Then we can have a look.
Kind regards
Philipp